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Document 1861671
34th Annual International Conference of the IEEE EMBS
San Diego, California USA, 28 August - 1 September, 2012
ASIC Design and Data Communications for the Boston Retinal
Prosthesis
Douglas B. Shire, Member, IEEE, William Ellersick, Shawn K. Kelly, Member, IEEE, Patrick Doyle,
Attila Priplata, William Drohan, Oscar Mendoza, Marcus Gingerich, Bruce McKee, John L. Wyatt,
Senior Member, IEEE, and Joseph F. Rizzo III, MD
Abstract²We report on the design and testing of a
custom application-specific integrated circuit (ASIC)
that has been developed as a key component of the
Boston retinal prosthesis. This device has been designed
for patients who are blind due to age-related macular
degeneration or retinitis pigmentosa. Key safety and
communication features of the low-power ASIC are
described, as are the highly configurable neural
stimulation current waveforms that are delivered to its
greater than 256 output electrodes. The ASIC was
created using an 0.18 micron Si fabrication process
utilizing standard 1.8 volt CMOS transistors as well as
20 volt lightly doped drain FETs. The communication
system receives frequency-shift keyed inputs at 6.78 MHz
from an implanted secondary coil, and transmits data
back to the control unit through a lower-bandwidth
channel that employs load-shift keying. The design's
safety is ensured by on-board electrode voltage
monitoring, stimulus charge limits, error checking of
data transmitted to the implant, and comprehensive selftest and performance monitoring features. Each
stimulus cycle is initiated by a transmitted word with a
full 32-bit error check code. Taken together, these
features allow researchers to safely and wirelessly tailor
retinal stimulation and vision recovery for each patient.
I. INTRODUCTION
T
HE retinal neural network is a part of the brain with
well-understood geographic organization; patients have
provided detailed feedback on its response to electrical
stimuli [1]. Advances in micro-fabrication, electronics,
materials, surgical techniques and research have enabled the
Boston Retinal Implant group (BRIP) to build an advanced
retinal prosthesis to restore vision to patients who have lost
Manuscript received March 29, 2012. This work was supported in part
by the U.S. Department of Veterans Affairs, Rehabilitation Research and
Development Division, through Center of Excellence grant C4266-C.
Additional support was provided by NIH grant EY016674 to Prof. Wyatt at
MIT, by the 16)¶V VXSSRUW RI WKH &RUQHOO 1DQR6FDOH )DFLOLW\ and Prof.
Wyatt, by the Massachusetts Lions Foundation, and by private sources.
D. B. Shire, S. K. Kelly, M. D. Gingerich, W. Drohan and B. McKee are
with the Boston VA Healthcare System, 150 S. Huntington Ave., Boston,
MA 02130 (phone: +1 (607) 339-7085; fax: (607) 255-8601; e-mail:
[email protected]).
O. Mendoza and J. L. Wyatt are with the Massachusetts Institute of
Technology, Cambridge, MA 02139; J. F. Rizzo and A. Priplata are with
the Massachusetts Eye and Ear Infirmary, Boston, MA 02114; and P. Doyle
is with Harvest Automation, Billerica, MA 01862 USA.
U.S. Government work not protected by U.S. copyright
their vision due to retinal diseases, especially retinitis
pigmentosa and age-related macular degeneration. In recent
years, the visual prosthetic community has expanded to more
than 20 groups worldwide [1]-[3]. The collective outcome
of testing (see, e.g., [4]-[5]) KDV SURYLGHG LPSRUWDQW ³SURRIof-FRQFHSW´ HYLGHQFH LQFOXGLQJ GHPRQVWUDWLRQ
RI
seemingly safe stimulation thresholds; 2) that severely blind
patients can reliably see phosphenes; 3) that modulation of
electrical stimulation alters percepts; 4) that object
orientation and movement can be detected; and 5) that
restoration of vision with an acuity of about 20/1000 is
possible, although this often requires head movement to scan
letters and considerable time to interpret the visual scene,
sometimes tens of seconds. One notable result from Retina
Implant AG in Germany was an implant recipient who could
read one week after implantation [6]. Collectively, these
results surely are encouraging, but fall short of the goal of
creating more natural, higher quality vision that would more
easily justify the risk of surgery and the cost of the device.
The need to achieve higher quality of restored vision has
motivated us as well as other teams [7]-[8] to develop
advanced devices, which our team chose to do prior to
performing long-term human implants. We report on the
application specific integrated circuit (ASIC) that forms the
heart of our new prosthetic, the wireless means used to
communicate data to and from our device, and the means for
processing input images gathered from a video camera
mounted on glasses worn by the patient. Our results will
pave the way toward human trials, and, we hope, a real
improvement in quality-of-life for implant recipients.
II. ASIC DESIGN AND TESTING
A. System Description
The core element of our retinal prosthesis is the ASIC that
serves as its communication and control center. It is designed
to receive information from an external controller, control
the electrode drivers, and transmit outbound data. The
µUHYHUVH¶ Welemetry includes monitoring of electrode voltage
waveform data to ensure the safety of the patient and the
long-term integrity of the electrodes, and data to support the
debugging of implanted devices or lab experiments.
Figure 1 shows a block diagram of the BRI2 ASIC. In the
upper left, a bandgap voltage reference (VRF) and an
292
external resistor allow the REF block to distribute constant
voltage and constant current references across the 5x5mm
System on Chip (SoC). The TMP block generates a current
proportional to absolute temperature that is used to sense
changes in die temperature. The PWR block regulates onchip power supplies, while the RCV and XMT blocks
communicate with the external controller using frequencyshift-keying
(FSK)
and
load-shift-keying
(LSK),
respectively. A synthesized digital control logic block
interfaces between the RCV and XMT blocks and the array
of electrode drivers (DRV), as well as the on-chip analog test
PXOWLSOH[RUV $767 WKDW IHHG GXDO $'&¶V DQG WKH
electrode voltage monitor (DRVMON) that checks for
charge buildup on electrodes. An on-chip memory (MEM)
FDSWXUHV WKH RXWSXW RI WKH GXDO $'&¶V RU DQ\ RI D ZLGH UDQJH
of other on-chip digital signals, with a configurable sample
rate and start time.
REF
IRF
vbg
VRF
izt4u
ibg4u
adcvcm
itmp
TMP
XMT
ATST
DAC
static sigs
vddh
gnd
vdd!
PWR
vss!
cmp*ref1
RCV
states
dyn. sigs SMP
atst[1:0]
MEM
ADC
ADC
MUXHVL
rclk
rxd
DRVMON
control ctl[N:0] DRV
logic
iOut[N:0]
txd
levels of electrode polarization. While allowing both highcurrent short pulses and low-current long pulses, the ASIC
has hardware-enforced charge limits, guided by our prior
electrode characterization work [5]. Configuration pins allow
the limits to be changed for compatibility with a range of
electrode sizes. Each electrode is grounded (shorted to the
case) for at least 200 µsec before any stimulus, and its
voltage is monitored to ensure that the electrode is fully depolarized, and monitored again after the initial W1 pulse to
ensure that electrode polarization is within safe limits (see
Figure 2). The VDDH and VSS power supplies prevent
excessively large voltages from being driven on electrodes.
In addition, the integrated ADCs periodically sample the
voltage waveforms on each electrode, sending back detailed
measurements to the external controller; this allows open and
short circuits to be detected, as well as changes in electrodetissue impedance and responses over time. To reduce
bandwidth requirements, an efficient message organization is
used. Forward and reverse channels operate simultaneously
while stimuli are ongoing. In fact, restricting the stimuli to
charge-balanced waveforms for safety also reduces the
degrees of freedom, and thus the number of bits needed to
describe the waveforms. Figure 2 shows a first command
received from the external controller (RxCommand1), which
initiates a set of stimulus pulses while the subsequent
RxCommand2 is being received. The commands specify the
time width W1 and the sign of current for the initial pulse,
along with the inter-phase width Wip and the complementary
pulse width W2. For charge balance, I2 = I1 * (W1/W2).
Fig. 1. BRI2 ASIC block diagram.
RxCommand1
TABLE 1
RxCommand2
BRI2 Implant ASIC Technical Specifications
W1
Ielectrode
Power: 6.78 MHz carrier; ~30 mW transmitted
Data: 565 Kbps inbound, 70 Kbps outbound
Electrode Current: 0 ± 127 A in 1 A steps
Stimulus Pulse width: 17.7 ± 4500 sec; up to +/- 8 V
Wip
W2
I1
I2
Fig.2. Electrode stimulus control and waveform.
Safety features include stimulus charge limits, error
checking on data transmission, comprehensive self-test and
performance monitoring, and configuration pins to change
operating modes or to lock out settings that might somehow
cause harm. This allows debugging and optimization of
performance in the lab, and management of risk. A robust
ACK-only protocol with strong error checking is used to
communicate with the implant ASIC. Each message from the
external controller can result in a set of stimulus pulses on a
subset of the electrodes, and generates a corresponding
response from the implant ASIC with chip and electrode
status, and any other requested data. Radio communication
errors are nearly eliminated by the use of 32-bit CRCs,
reducing the probability of accepting a corrupted message to
2-10. No individual message can cause harm, and messages
with a bad CRC merely result in missing stimulus pulses.
Several features also work to ensure operation with safe
Any subset of electrodes can be driven in a given stimulus
cycle, and electrodes can be configured as sources of current
or as local current sinks (returns) to provide current steering
capabilities. There is considerable flexibility in pulse
repetition rate, and a 60 Hz refresh of all electrodes can be
wirelessly commanded as needed. Power consumption is
minimized in a number of ways, which ultimately limits the
RF energy the recipient must be exposed to while the implant
is active. Analog bias currents are typically 1µA, and are
distributed to local current mirrors shared by the electrode
drivers, which consume no power during the majority of the
time that they are not enabled; brief power-up transients are
charge-matched to first order between positive and negative
voltage sources.
The ability to connect almost any analog node, including
all electrodes, to on-FKLS $'&¶V IRU ZDYHIRUP WUDQVPLVVLRQ
back to the external controller, or to an on-chip DAC,
enabled testing of much of the analog circuitry. A similar
ability to capture or drive many digital signals, along with a
293
built-in scan test, also allowed faults to be detected.
A micrograph of the BRI2 retinal implant die is shown in
Figure 3. Part of the electrode driver array is in the upper
two thirds of the image, and centralized bias circuitry is at
the lower right; both are under a power and signal grid in the
transmitted power dropped off by 10% [9]. A resonant
capacitor was included on a small board inside the titanium
enclosure, along with Schottky diodes and capacitors to halfwave rectify a positive and negative supply relative to GND.
Since our charge-balanced stimulus drew equal currents from
VDDH and VSS, the dual half-wave rectification was
efficient, and minimized discrete components within the
package. Circuitry in the ASIC regulated VDDH and VSS to
minimize power and provided accurate stimuli. The implant
power supplies can run in two modes, +/- 8V, and +/- 4V.
These relatively high voltages are dropped mostly across
tissue; this is acceptable because of the low field due to the
large distance to the GND return.
EXTXMT
VDDH
GND
Fig. 3. Partial, wire-bonded die photo showing some of the >256 electrode
drivers, control logic, and analog circuits.
VSS
Fig. 5. BRI2 ASIC radio and power system block diagram.
WKLFN WRS PHWDO OD\HU 7KH GLH ZDV IDEULFDWHG LQ ,%0¶V KLJKvoltage 0.18µm CMOS process, with 20V LD MOSFETs.
B. Verification: ASIC, Power and Data Transmission
Bench testing of the ASIC using series RC electrode
models was performed (see Fig. 4). The lower trace in
Figure 4 represents an end-of-stimulus pulse, upon which the
electrode is shorted to the current return (GND) that is
connected to the platinum-coated, laser-welded Ti package.
Fig.6. Secondary coil, VDDH and VSS with simultaneous FSK and LSK.
Fig. 4. Upper trace: typical output of the HD retinal micro-stimulator ASIC
when powering an electrode modeled as a series RC circuit. Lower trace:
an end-of-stimulus pulse, upon which the electrode is shorted to ground.
Figure 5 shows the external transmitter (EXTXMT) with
its Class E power amplifier connected to a resonant
capacitance in parallel with a 7µH primary coil (mounted in
glasses that patients will wear). The 4.4µH secondary coil,
implanted around the cornea, is approximately 1cm from the
glasses, with coupling factor k ~ 0.14. Previous testing by
our group showed that at a typical maximum angular
displacement from normal alignment of the primary and
secondary coils of 15 degrees due to eye movement, the
Fig. 7. Wireless data transmission to the retinal implant ASIC.
Figure 6 shows the voltages on the secondary coil, VDDH
and VSS supplies during power-up of the BRI2 ASIC. The
high frequency AC is occasionally attenuated at the ASIC to
transmit data back to the external controller using load-shiftkeying (LSK). Somewhat counter-intuitively, LSK that
attenuates the secondary coil voltage resulted in a larger
signal at the primary coil (in the external glasses), as less
294
power is drawn from the coupled resonant system. Different
attenuation strengths are shown, which allow for
optimization of data and power transmission. In addition, the
power supplies can be seen to be charging up from +/-4V on
the left to +/-4.2V on the right, as power is drawn from the
secondary coil via the Schottky diodes.
A final test was to transmit data to the chip using the RF
transmitter. Data reception is demonstrated in Fig. 7, in
which the top trace represents WKH FKLS¶V VDPSOLQJ FORFN, the
middle waveform represents the received data (which is valid
at the rising edge of the sampling clock), and the bottom
trace represents the data fed to the transmitter board.
C. Video Image Pre-Processing
We are currently developing image processing software
for the retinal implant and the user interface that patients and
clinicians will use together to adjust video pre-processing to
achieve the best visual percepts. Images are captured by a
glasses-mounted camera at 30 frames/sec and then sent to the
external controller, where they are loaded into memory as a
640 x 480 color (RGB) image. They are then converted to
grayscale (7-bit) and sent through a series of user-defined
filters, chosen e.g. from contrast enhancement, edge
extraction, blurring, and thresholding. After filtering, the
image is down-sampled to >256 points, one for each
electrode. 7KH FDPHUD¶V automatic gain control compensates
for varying ambient light levels. The choice of which
algorithms to use and what parameters to set for each filter is
likely to vary across patients, across different lighting
environments, and may also depend on the task that the user
is performing. Figure 8 shows an illustration of what video
processing might look like. The raw image is sent through a
In the portable version of our device, we will implement
these algorithms using a dedicated DSP to achieve sufficient
computation speed in the embedded system. We anticipate
that optimized patient input for image processing algorithms
(and stimulation parameters, within safe limits) will be
determined in an iterative fashion.
III. CONCLUSIONS AND FUTURE WORK
We have designed and extensively bench-tested a highly
configurable, high-density neuro-stimulator ASIC in both
wired and wireless configurations. This >256 channel device
is appropriate for chronic implantation with our proven,
minimally-invasive sub-retinal surgical implantation
techniques, and its sophisticated LSK reverse telemetry
features will enable optimization of stimuli for each patient.
Extensive safety features have been implemented in this
chip; driving software, GUI development, and image
processing algorithms for the external system are under
development, as we prepare for pilot human trials of our
retinal prosthesis.
ACKNOWLEDGMENT
The authors thank W. Hansford and MOSIS, M. Lugin
and M. Segien for custom layout work, G. Galanek and J.
Dumser for administrative assistance, the Cornell Nanofabrication staff, and S. Behan for his engineering expertise.
REFERENCES
[1]
[2]
[3]
[4]
[5]
FILTER SELECTIONS:
Blurring
Edge Extraction
Thresholding
Contrast Enhancement
Zoom In
Rotate Image
Linear (e.g., low-pass)
[6]
Fig. 8. Video filtering and down-sampling for the retinal implant.
[7]
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is spatially aligned with the position of an electrode. Using
our user interface, each filter can be turned on or off, and its
parameters can be adjusted. The white box in the top left
image of Fig. 8 represents a zoom window, allowing the user
to zoom in or out on a scene. Currently, the image processing
algorithms are implemented on a personal computer running
C++/µOpenCV¶. Depending on the extent of the image
processing, we achieve frame rates from 5 to 20 frames/sec.
[8]
[9]
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